1. Field of the Invention
The present invention relates generally to analysis of the chemical composition of the surface layers of an object. More particularly, this invention relates to a system for thermodynamically separating evolved gases to allow more effective detection and analysis thereof. Still more particularly, the present invention relates to a method and apparatus for evaporating some components from the surface of a semiconductor wafer, condensing and separating the evolved gases, and then analyzing the condensates.
2. Description of the Relevant Art
During the course of manufacturing an integrated circuit, films or layers of various substances may be applied to the surface of a silicon wafer. Some of the substances are intended to remain on the wafer surface, while others merely serve as sacraficial masking layers (photoresists), and are ultimately removed. In addition, the surface of the wafer may be doped with various substances. At some point during chip manufacture it may be desired to test the wafer and the overlying film(s), in order to ascertain both the presence of desired substances and the absence of undesired substances.
This analysis can be done by known techniques, such as infrared spectroscopy or thermogravimetric analysis (TGA). Neither technique is entirely satisfactory, however. Infrared spectroscopy is not particularly effective for detection of silicates, which have little infrared activity. Thermogravimetric analysis involves measuring the change in weight of an object as it is heated at a controlled rate, and is therefore limited in the amount of detail it can provide. For example, when used with organic materials, TGA gives insufficiently detailed information because each organic compound may fracture into two or more components, resulting in a wide range of organic molecules and making analysis of the weight data difficult. In addition, some known analytical techniques can damage the wafer, while others require destruction of the wafer.
At present, testing is often accomplished by applying heat to the wafer being tested, thereby causing some of the compounds on or near the surface of the wafer to volatilize. The resulting gas can be collected and fed into a mass spectrometer or other analytical equipment. Because many of the evolved species have similar properties, however, it is difficult to interpret the resulting spectrum. Furthermore, the gas species evolved according to this method are not monoenergetic (i.e., they show an array of energies according to a normal distribution), and therefore do not give an entirely accurate reading as they pass through the mass analyzer. Finally, only small amounts of any species are evolved, further confounding attempts at analysis.
Hence, it is desired to proved an analytical technique that allows semiconductor wafers to be effectively analyzed without destruction of the wafer. The desired technique has a minimal structural or electrical effect on the wafer and provides an easily analyzable array of the compounds present at or near the surface of the wafer.